STRATIGRAPHIC AND GEOCHRONOLOGIC CONSTRAINTS ON TIME-SPACE PATTERNS OF LATEST QUATERNARY SURFACE RUPTURES ON THE EASTERN PINTO MOUNTAIN AND SOUTHERN MESQUITE LAKE FAULT ZONES NEAR TWENTYNINE PALMS, SOUTHERN CALIFORNIA

Christopher M. Menges, & Shannon A. Mahan

Published August 15, 2019, SCEC Contribution #9879, 2019 SCEC Annual Meeting Poster #109

Recent surficial geologic mapping and new luminescence dating provide constraints on the time-space patterns of latest Quaternary surface ruptures along the E Pinto Mountain fault (PMf) and S Mesquite Lake fault (MLf) near Twentynine Palms in southern California. The E-trending sinistral PMf and the NW-trending dextral MLf intersect one another in the study area. Our interpretations are based primarily on the post-IR IRSL ages because of documented issues with the application of quartz OSL of deposits in the Mojave Desert.The PMf is an E-trending left-lateral fault that increases in complexity from one to four branches, and changes average orientation to the southeast, towards the eastern end of the mapped fault zone Three sites are located among two adjacent branches on southern margin of the fault. At these sites surface ruptures displace or are buried by Holocene deposits between 5.14 and 1.27 ka. At two of these localities, more than three individual fault traces displace a stacked series of four alluvial deposits at three stratigraphic levels. The lowermost deposit exposed near the base of the section has a latest Pleistocene age (~12.31 ka) and bears stratigraphic evidence of a minimum of three surface ruptures. The most recent event (MRE) at these two sites is bracketed by two faulted late and mid Holocene units (~1.41 and ~5.14 ka) and an overlying undisturbed deposit with a late Holocene age (~1.27 ka). Farther to the southeast, three late Holocene alluvial fan deposits between ~1.40 and ~3.13 ka display a series of remnant depositional bars and swales that are sinistrally offset by 3-4 m across a one strand of the PMf by a single surface rupture related to the MRE at the sites to the west. Many late Pleistocene alluvial fans with numeric ages between ~15.01 and ~49.59 ka, are highly deformed and laterally displaced ≥ 20 m on all branches of the PMf.The MLf is a NW-striking dextral strike-slip fault that traverses eolian-dune and alluvial-fan deposits southeastward across the map area as it is approaches intersection with the PMf. . Sheared eolian-dune sand with a late Holocene date (~1.27 ka) is observed at the base of faulted margin of a sag playa on the MLf in the northern map area. To the SE several nested alluvial fans (~1.97 ka and ~2.28 ka) are right laterally offsets by 3-4 m. These zones of surface rupture are likely related and define the MRE on this part of the MLf. similar to the MRE defined by our chronology for the PMf.

Key Words
Quaternary faults, Surface Ruptures, Event timing and location

Citation
Menges, C. M., & Mahan, S. A. (2019, 08). STRATIGRAPHIC AND GEOCHRONOLOGIC CONSTRAINTS ON TIME-SPACE PATTERNS OF LATEST QUATERNARY SURFACE RUPTURES ON THE EASTERN PINTO MOUNTAIN AND SOUTHERN MESQUITE LAKE FAULT ZONES NEAR TWENTYNINE PALMS, SOUTHERN CALIFORNIA. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology